A data link, data bus, or data network is an array of data transmitters and receivers arranged to transmit data bi-directionally between two or more users over wire, optical fiber, or another transmission medium. Diagnostic instruments that monitor, process, and display the data communicated over these links, buses, or networks are necessary for integrating and trouble shooting complex systems. A bus monitor is one such diagnostic instrument that receives data transmitted over a data link, data bus, or data network and extracts selected information to be displayed or otherwise used. For example, the bus monitor may be connected to a host computer that displays the data in various formats to aid in solving various integration problems. In military aircraft applications, a bus monitor is commonly used as flight test instrumentation to verify data bus performance and/or the performance of a subsystem attached to the bus.
One type of data bus used in aerospace applications is the SAE AS1773 20 Mbps multiplexed command/response fiber-optic data bus. As is known to those skilled in the art, MIL-STD-1773, entitled "Fiber Optics Mechanization of an Aircraft Internal Time Division Command/Response Multiplex Data Bus," is a 1 Mbps fiber-optic version of the popular 1 Mbps copper wire MIL-STD-1553 data bus, and provides the option of using fiber-optics in military avionics systems in place of MIL-STD-1553. MIL-STD-1553 defines a serial asynchronous data bus employing Manchester encoding on which the messages are time multiplexed among users, allowing several users (nodes) to share the same resource (transmission medium) by using it at different times. MIL-STD-1773 specifies a 1 Mbps protocol that is identical to MIL-STD-1553, with the difference confined to the transmission medium. MIL-STD-1773 specifies a fiber-optic medium, while MIL-STD-1553 specifies an electrical medium. AS1773 includes both 1 Mbps and 20 Mbps protocols. The 20 Mbps fiber-optic AS1773 data format is identical to the 1 Mbps format, except that the rate of data transmission is substantially higher, and the AS1773 protocol differs in having a preamble before each message to aid in recovering the clock at the higher data rate. The AS1773, MIL-STD-1773, and MIL-STD-1553 data buses all use Manchester encoded data (Manchester II bi-phase level) to send messages of up to 33 words, each consisting of 16 data bits followed by a single parity bit and preceded by a characteristic synchronization pulse that consists of an easily recognizable illegal Manchester pattern.
Use of the 20 Mbps AS1773 bus has heretofore been limited because there is no conventional diagnostic instrument available to provide a monitoring function for this high speed fiber-optic data bus. Such test instrumentation is necessary to integrate equipment into applications, for example aircraft control systems. The 20 Mbps AS1773 bus provides a twenty times speed advantage over the popular MIL-STD-1553 bus and the additional advantages of being immune to Electro-Magnetic Interference (EMI), as well as not being a source of EMI. The AS1773 bus was developed primarily for space applications, so the components are designed to provide the additional benefit of being immune to single event upsets due to ionizing radiation. In order to increase the ease of integrating the 20 Mbps AS1773 data bus into various applications, it would be desirable to provide a system and method for monitoring a high-speed data bus of the serial, asynchronous type employing Manchester encoding, which provides a means to monitor the data transmitted at such a high data rate. Preferably, the system would retrieve messages from the data bus and store the messages for processing and analysis.